Add easy way to handle transfer of gamma / transcriptions from one account to another

[itsthejoker]

Right now, this can be done through shell quite easily:

old = BlossomUser.objects.get(username="old_account")
new = BlossomUser.objects.get(username="new_account")

transcriptions = Transcription.objects.filter(author=old)
for T in transcriptions:
  T.author = new
  T.save()

But there should be either a graphical interface method for this or a way to do it using the slack connector.

[TimJentzsch]

Closing this in favor of #165.